Spherical inertial electrostatic confinement device as a tunable x-ray source

ABSTRACT

A low cost small-scale tunable X-ray source, comprising a spherical-electron injected inertial electrostatic confinement (IEC) device. Within a spherical containment vessel (402) recirculatory focusing electrons are accelerated by a spherical grid (401) within the vessel, and cause electron-electron collisions in a dense, central plasma core region (404) of the sphere. The IEC synchrotron source (IEC-SS) in a mechanism for generating tunable X-ray radiation is essentially equivalent to that for conventional synchrotron sources. The IEC-SS operates at a much lower electron energy (&lt;100 kev compared with &gt;200 Mev in a synchrotron), but still gives the same X-ray energy due to the small-scale bending radius associated with the electron-electron interactions. The X-rays can be filtered for particular purposes using diffraction gratings, prisms or the like.

This application claims domestic priority from U.S. ProvisionalApplication Ser. No. 60/030,009 filed Nov. 1, 1996, and the entire econtent of that application is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The development of a compact, tunable, hard x-ray source would haveprofound and wide ranging applications in a number of areas. These areasinclude x-ray diagnostics, medical imaging, microscopy, nuclearresonance absorption, solid-state physics and material science.

Currently, varieties of x-ray generators exist. The most modern devicesare generally based on one of three methodologies: laser and dischargeplasmas, electron impact sources, and synchrotron. The spectrum of thesesources can be divided into two categories: characteristic x-rays andcontinuum x-rays. The characteristic x-ray sources are dependent on theparticular atomic structure of the gas or target material in use. Amongall the types of x-ray sources, only synchrotron produces continuumradiation.

The main interest in laser-generated plasma is directed towards inertialconfined fusion. Recently, they have also gained interest as sources of(V)UV and x-rays. Laser-generated plasmas emit photons in an energyrange, which can extend from visible light to hard x-rays. The observedemission spectrum is characteristic of a high-temperature, short-lived,high-density plasma. The sources produce a spectrum of x-rays centeredabout characteristic lines of the material.

In a laser-generated plasma x-ray source, when a high-power pulsed laseris focused on a (solid) target, a plasma is created. After the laserpulse terminates, the plasma cools extremely rapidly due to rapidthermal conduction, electron energy loss to ions, and expansion of theplasma into the surrounding vacuum. Cooling of the electrons at highdensity leads to fast recombination, quenching of the highly excitedstates, and a termination of the x-ray emission. The choice of targetmaterial controls the intrinsic range of the spectral output determinedby the ionization states of the target material. Details of the spectraldistribution are highly dependent on the target material (e.g., carbon,aluminum, titanium, copper, zinc, molybdenum, tin, tungsten, and lead)and other parameters (target thickness and source size).

Plasma discharge systems have been suggested as sources of highbrightness x-ray radiation. Most of these devices (the gas puff J.Pearlman an J. C. Riordan, J. Vac. Sci. Technol. 19, 1190 (1981), plasmafocus Y. Kato, et al, Appl. Phys. Lett. 48,686 (1986), and hypocycloidalpinch K. S. Han, et al, Bull. Am. Phys. 31, (1986)) are variations ofthe Z-pinch geometry. In Z-pinch devices, a high current is produced onthe outer edge of a cylindrical volume of gas using a pulsed electricaldriver such as a fast capacitor bank. The resulting JxB forceaccelerates the plasma shell radially inward to form a veryhigh-temperature plasma on-axis which emits characteristic thermalradiation in the soft x-ray region.

The conventional electron impact sources use a suitable target materialthat is bombarded by a high-energy electron beam. These sources producea broad spectrum of x-rays centered about characteristic lines of thematerial.

Synchrotron radiation is the electromagnetic radiation emitted byelectrons moving at relativistic velocities along a curved trajectorywith a large radius of curvature, for example, several meters to tens ofmeters. The energy of the photons ranges from a few electron volts to10⁵ Ev. This corresponds to the binding energy of electrons in atoms,molecules, solids, and biological systems. Thus, synchrotron radiationphotons have the right energy to probe the properties of such electronsand of the corresponding chemical bonds to understand their physical andchemical properties. The uses of electron accelerators as sources ofsynchrotron radiation have grown enormously during the last two decades.Unique features such as tunability and wide x-ray spectrum tend torender the synchrotron irreplaceable for many applications.

Presently, third generation synchrotron sources are being pursued thatare based on high-energy electron storage rings and bending magnets. Atypical electron accelerator can be tuned to emit synchrotron radiationin a very broad range of photon energies, from microwaves tohard-x-rays. Thus, it provides electromagnetic radiation in spectralregions for which no other usable sources exist, e.g., most of theultraviolet/soft-x-ray range. Furthermore, it is by far the best sourceof hard-x-rays, even though other sources exist for this range. Thesystem has met most application needs, but fails with respect tophysical size and cost. They are inevitably large and expensive devicesrequiring complex supporting facilities. The current machines are verylarge and costly with tens to hundreds of millions of dollars. Thenature of synchrotron x-ray sources means that they are expensive,remote multi-user facilities, and are therefore not suited for use witha laboratory scale. The alternative x-ray sources, such as electronimpact systems, laser and discharge plasmas, cannot match synchrotron interms of its tunability and continuum x-rays.

An object of the invention disclosed is to provide a small compacttunable x-ray source.

Another object is to provide a compact tunable x-ray source forlaboratory use. For applications where a relatively small sample ispractical, the availability of a laboratory-scale source would be veryadvantageous.

Another object is to provide a compact tunable x-ray source for securityinspection applications such as more sensitive balcale x-ray inspectionsystems.

SUMMARY OF THE INVENTION

A low cost, compact, tunable x-ray source, that is based on an inertialelectrostatic confinement (IEC) vessel design, is proposed. The IECdevice is described in pending U.S. patent application Ser. No.08/232,764 for “Inertial-Electrostatic Confinement Particle Generator”and Ser. No. 08/491,127 for “Electrostatic Accelerated RecirculatingFusion Neutron/Proton Source” which are incorporated herein byreference.

In the IEC-based x-ray source design, the electron storage ring of thesynchrotron is replaced by recirculatory focusing electrons in a spherethat are accelerated by a grid, and the bending magnets are replaced bythe electron—electron collisions in the sphere center. This arrangementresults in an IEC synchrotron source (IEC-SS), wherein the mechanism forgenerating tunable x-ray radiation is essentially the same as in thebending magnet synchrotron sources. The IEC-SS operates at a much lowerelectron energy (<100 keV compared with >200 MeV in a synchrotron) whilestill giving a same radiated x-ray energy compensated by a bendingradius of much smaller scale from electron—electron interactions. Inshort, electrons are accelerated 10's to 100 kev by the anode grid. Dueto spherical (or other) convergence, the energetic electrons scatter inthe center of the sphere. The scattering interactions create intensebremsstrahlung x-rays. The emitted x-ray energy is controlled by thegrid bias.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an IEC device.

FIG. 2 is a schematic illustration of an IEC-SS x-ray source inaccordance with the present invention.

FIG. 3 is a measured X-ray spectrum.

FIG. 4 is an intensity vs wavelength graph pertinent to the presentinvention.

FIG. 5 is an intensity vs wavelength graph pertinent to the presentinvention.

FIG. 6a is an illustration of a vacuum port target holder arrangement.

FIG. 6b is an illustration of an x-ray window-external targetarrangement.

FIG. 7 is a schematic illustration of a pin hole camera system for x-rayimaging using the x-ray source.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An inertial electrostatic confinement (IEC) particle generator isdescribed in U.S. patent application Ser. No. 08/232,764 (Miley et al.)which was filed on Apr. 25, 1994 and is incorporated herein byreference. The inertial electrostatic confinement device disclosedtherein includes a vacuum vessel which is held at ground potential andcontains internally and concentric to the vessel, a wire grid which actsas a cathode. The cathode may be made from a variety of metals havingstructural strength and appropriate secondary electron and thermionicelectron coefficients. The cathode wire grid is connected to a powersource to provide a high negative potential (30 kV-70 Kv), while thevessel itself is conductive and maintained at a ground potential.Deuterium or a mixture of deuterium and tritium gas is introduced intothe vessel. A voltage is applied to the cathode wire grid and thepressure is adjusted in order initiate a glow discharge. To maximize theneutron yield per unit power input while maximizing grid life-time byreducing collisions with a grid, operational conditions are used tocreate a “star” glow discharge mode. The glow discharge generates ionswhich are extracted from the discharge by the electric field created bythe cathode grid. These ions are accelerated through the grid openingsand focused at a spot in the center of the spherical device. Theresulting high energy ions interact with the background gas(beam-background collisions) and themselves (beam-beam collisions) in asmall volume around the center spot, resulting in a high rate of fusionreactions. The result is a neutron generator producing neutrons as oneof the D-T fusion reaction products. Where the ejection rates are high,the ejected ions may provide a deep-self generated potential well thatconfines trapped beam ions, creating even higher reaction rates. Thedevice may be modified by using a field gas mixture of deuterium andhelium-3 to be a source of protons as well as neutrons. One geometricalform of the device is spherical and as seen in FIG. 1. This device isbased upon the principle of an ion accelerator with a plasma target. Ina neutron-generator embodiment, deuterium-deuterium fusion reactionstakes place in the plasma target and produce energetic neutrons. Thedevice acts as a simple spherical plasma diode, having a groundpotential on the outer sphere and a negative potential on a nearlygeometrically transparent inner spherical grid. The spherical inertialelectrostatic confinement device 10 is illustrated in FIG. 1 where aconductive vacuum chamber 11 is connected to a ground potential atcontact 17. The device has a cathode grid 12 which defines a smallsphere within the chamber and has a grid design that provides a highgeometric transparency. In operation, however, this grid design has aneven higher effective transparency, due to the effect of a concentrationof ions into a“microchannels”, as subsequently described. A source ofpower 14 is connected by a high voltage feed through to the internalcathode grid 12. The voltage has a negative value, thereby providing abias between the relatively positive walls of the vacuum chamber and thecentral grid area. Gas is introduced into the vacuum chamber 11 by acontrol valve 15 and is evacuated by a pump 18.

Upon application of a potential to the cathode grid, under certaingrid-voltage, gas pressure, gas type and grid-configuration conditions,high density ions and electron beams will form within the IEC deviceinitiating a “star” mode of operation. In this mode, high density spacecharged neutralized ion beams are formed into microchannels that passthrough the open spaces between the grid wires. As the ions avoidcontact with the wires, this mode increases the effective gridtransparency to a level above the geometric value. These microchannelssignificantly reduce grid bombardment and erosion and increase powerefficiency. For conventional star mode operation, the grid andmicrochannel beams are symmetric so that a convergent high-density coredevelops. The inertial electrostatic confinement device serves as avaluable source of neutrons or protons.

The spherical inertial electrostatic confinement (IEC) device has beenused as a plasma fusion reactor. In a plasma fusion reactor, the energyproduction must compete with inevitable losses, and the role of theprocesses which result in such losses is crucial in determining theoperating temperature of a plasma fusion reactor. Some energy losses canbe minimized by a suitable choice of certain design parameters, butothers are inherent in the reacting system; one of these isbremsstrahlung radiation. The efficiency of neutron production competeswith the inevitable losses of bremsstrahlung radiation that are inherentin the reacting system. High intensity x-rays were measured inexperiments Hirsch's x-ray measurement. Previously, the goal was tominimize the bremsstrahlung radiation by a suitable choice of certaindesign parameters. Affirmative use of this property can permit a deviceto serve as x-ray source.

An IEC plasma x-ray source may have the general structure as seen inFIG. 2, wherein electrons are injected into the center of a sphericalIEC device 400, formed from two spherical concentric electrodes. Theinner electrode 401 (anode) made of a highly transparent grid (>90%,preferably >95%, transparency) is charged to a positive voltage,preferably in a range of 1 kV to 150 KV, relative to the outer groundedelectrode 402 (cathode), at driving currents varying from 1 mA to 100mA. The outer electrode is a hermedically sealed vacuum chamber thatsupports a pressure of less than 10⁻⁶ Torr. Electrons emanating from thecathode 402 are attracted to the anode 401, and pass through the anode(grid) many times before being captured by the grid. Due to sphericalconvergence, the injection of electrons constitute an accumulation ofelectrons that forms a dense electron cloud which then can be used toaccelerate and heat ions. The electrons are injected by electronemitters 409 which are electrically heated to generate the electrons.There are at least two, preferably four to eight, such assemblies, andeach assembly is comprised of an electron emitter and an electronextractor. The operation generates intense bremsstrahlung radiation inthe spherical center due to the strong electron—electron interactions ata relativistic speed accelerated by the grid bias. The energy spectrumof the emitted x-rays shifts as the grid bias is changed. Notably, thebias on this configuration is opposite to that seen in FIG. 1, whereinthe central grid is a cathode and the chamber 11 serves as an anode.

As is well known, the plasma in a thermonuclear reactor consists ofstripped nuclei of hydrogen isotopes together with electrons. From sucha plasma, energy will inevitably be lost in the form of bremsstrahlung,that is, radiation emitted by charged particles, mainly the electrons,as a result of deflection by the Coulomb fields of other chargedparticles.

An expression for the rate of electron-ion bremsstrahlung energyemission of the correct form L. Spitzer, USAEC Report NYO-6049 (1954),P. 9., but differing by a small numerical factor from the resultobtained by a more rigorous procedure, can be derived from the classicalexpression for the rate P_(e) at which energy is radiated by anaccelerated electron, namely, $\begin{matrix}{P_{e} = {\frac{2\quad e^{2}}{3\quad c^{3}}\quad a^{2}}} & (1)\end{matrix}$

where e is the electron charge, c is the velocity of light, and a is theelectron acceleration. The total power P_(br) radiated as bremsstrahlungper unit volume has been calculated in a Maxwellian distribution ofvelocity among the electrons in a system containing a single ionicspecies of charge Z. S Glassston and R. H. Lovberg ControlledThermonuclear Reactions, Van Nostrand Reinhold Company, 1960, Chapter 2.$\begin{matrix}{P_{hr} = {\frac{16\quad \pi^{2}}{3^{1/2}}\quad \frac{\left( {kT}_{e} \right)^{1/2}e^{6}}{m_{e}^{3/2}\quad c^{3}h}\quad n_{e}{niZ}^{2}}} & (2)\end{matrix}$

where T_(e) is the kinetic temperature of the electrons in a Maxwelliandistribution, n_(e) and n_(i) are the density of electron and ion,respectively, m_(e) is the electron rest mass, and h is Planck'sconstant. The classical expression for the rate of bremmstrahlungemission per unit volume per unit frequency interval in the frequencyrange from v to v+dv is $\begin{matrix}{{dP}_{v} = {\frac{16\quad \pi^{2}}{3^{1/2}}\quad \left( {kT}_{e} \right)^{1/2}\quad \frac{e^{6}}{m_{e}^{3/2}\quad c^{3}}\quad n_{e}{niZ}^{2}{\exp \left( {{- {hv}}/{kT}_{e}} \right)}{{dv}.}}} & (3)\end{matrix}$

Upon integration over all frequencies, this expression leads to equation(2). For arbitrary electron and ion densities, the equation (3)expressed in terms of wave length, the relative values of Dpλ,/dλ havebeen plotted as a function of wave length in FIG. 4 (From C. T. Ulrey:Phys. Rev., 11:401 (1918), as cited on page 616, Evans, The AtomicNucleus, McGraw-Hill, Inc., (1972). While this calculation was performedfor a thick tungsten target, the shape of the spectra is expected to bequite similar to that obtained from the IED due to the similarity of thex-ray production mechanisms. To the left of the maximum for each curve,the energy emission as bremsstrahlung is dominated by the exponentialterm and decreases rapidly with decreasing wave length. Thebremsstrahlung power distribution is calculated assuming a Maxwellianelectron velocity distribution. For monoenergetic electron velocity, thedistribution is expected to be narrower.

At temperature below 50 kev, the bremsstrahlung from a plasma arisesalmost entirely from electron-ion interactions. At high temperatures,the production of bremsstrahlung due to electron—electron interactions,as distinct from those resulting from the electron-ion interactions,will be significant. Provided relativistic effects do not arise, thereshould be no electron—electron bremsttrahlung, but at high electronvelocities such is not the case and appreciable losses can occur fromthis form of radiation. The following results will provide a generalindication of the situation. At an electron kinetic temperature of 25keV the ratio of electron—electron bremsstrahlung energy to that forelectron-ion interaction is estimated to be 0.06, at 50 keV it is 0.13,and at 100 keV it is 0.34. C. F. Wandel, et al, Nuclear Instr., 4, 249(1959). R. F. Post, Ann. Rev. Nuclear Sci, 9, 367 (1959).

In the IEC configuration, under proper conditions ofcurrent-voltage-pressure, a virtual cathode can form. [G. Miley et al,Inertial-Electrostatic Confinement Neutron/Proton Source, AIP conf.proc. 299. Editors: M. Haines, A. Knight.] In that case, deceleration ofthe electrons as they approach the virtual cathode makes an additionalcontribution to the x-ray yield. [R. Eisberg, Quantum Physics of Atoms,Molecules, Solids, Nuclei, and Particles, 2nd Ed., John Wiley and Sons,1985.] This term can equal or dominate the electron/electron collisionalcontributions, depending of the height of the virtual cathode. Sinceelectrons can lose their entire energy x-rays in this case, the effectgenerally causes a shift of the x-ray spectrum to higher energies.

Experimental measures of the x-ray spectrum have been carried out usingthe experiment setup described in FIG. 2. Results are shown in FIG. 3.As expected, the data follows along a curve very similar to calculatedspectra, previously shown in FIG. 4.

The IEC spectrum in FIG. 3 was taken with the applied voltage set at 30kV. The measured spectrum is somewhat broad having a 15 keV full-widthat half-maximum (FWHM) for a spectrum ranging up to 230 kV (comparableto a 12 keV FWHM for E-30 keV in FIG. 4). The peak of the distributioncan be shifted by varying the applied grid voltage to give a series ofspectra similar to that of FIG. 4. For many experiments, a broad-rangespectrum of this nature is quite useful. However, in some cases it maybe desirable to employ a narrow band of x-ray energies.

If so, a narrower spectrum or “band” can be selected by Bragg reflectionfrom crystal surfaces, or by diffraction gratings, or by using other“conventional” x-ray optics techniques (J. B. Murphy et al.,“Synchrotron radiation resources and condensers for projection x-raylithography,” Appl. Optics, vol. 32, no. 34, pp. 6920-6929 (Dec. 1,1933); I. A. Artyukov et al., “On the efficiency of grazing incidenceoptics: the spiral collimator,” in Short Wavelength Lasers and TheirApplications, Nova Science Publishers, Inc., N.Y., pp. 299-310 (1992);H. Takenaka et al., “Heat resistance of Mo-based and W-based multilayersoft x-ray mirrors,” in Laser Interaction and Rolation Plasma Phenomena,12 International Conference, Osaka, Japan 1995, Part II, AmericanInstitute of Physics, pp. 808-813 (1992).) Such x-ray band selection isespecially desirable in certain types of experiments or industrialapplications where a narrow range of x-ray energies is desired. By usingband selection techniques, the IEC voltage is first tuned to optimizethe overall x-ray spectrum in the range desired. The x-ray band selectoris then employed to further narrow the range of x-ray wavelengthsstriking the target or spectrum under treatment. This process isillustrated in FIG. 5. Assuming that x-rays in the wavelength range0.45-0.55 nm are desired, the IEC voltage is first raised to 50 kV. Thisshifts the maximum intensity of the broad x-ray spectrum such that, asseen in the figure, the peak lies over the desired range. Then, anappropriate band selection technique (diffraction grating, etc.) isemployed to select the 0.45-0.55 nm band. As observed from the figure,this procedure, adjusting the IEC x-ray spectrum followed by bandselection, optimizes the x-ray intensity obtained in the desired range.If the IEC voltage had not been optimized, e.g., left at 30 kV or lower,the figure shows that the intensity in the desired band would be reducedby 50% or more.

Otherwise, if a narrow wavelength of x-rays is not required, the tunedIEC x-ray can be used directly.

Coupling of the band selection optics to the IEC x-ray source can beaccomplished in a variety of ways. Two characteristic methods,illustrated in FIGS. 6a and 6 b, differ by inserting the selectionoptics and target inside the IEC vacuum chamber 11, or using externaloptics 501 with x-rays extracted from the vacuum vessel through a thin,vacuum-tight, metallic x-ray window 502. FIG. 6a uses “conventional”x-ray diffraction optics 451 (C. V. Azaroff, X-ray SpectroscopyMcGraw-Hill, N.Y., (1973).) for band selection. It and the target 452are located in an expanded port 453 on the side of the IEC. The port 453is connected through an opening 404 in the main vacuum vessel such thatx-rays escape the IEC grid region and enter the optics system while theport volume is maintained under vacuum conditions through the mainchamber pumping system. A double valve 455 arrangement on the end of theport allows convenient insertion and removal of targets/specimenswithout breaking the main chamber vacuum. This method has the advantagethat the x-rays escaping the IEC are not attenuated by use of a vacuumwindow (such as in FIG. 6b), and the target can be maintained undervacuum conditions. On the other hand, insertion and removal of thetarget/speculum through the double gate valve system is a complication.If a slightly reduced x-ray intensity is tolerable, and if the targetneed not be maintained under vacuum, the external arrangement of FIG. 6bcan be used. Here x-rays from the IEC chamber 11 escape through a low-Zmetallic window 502. A low-Z material such as Be would be used tominimize x-ray attenuation which maintaining structural strength to holdvacuum conditions. Select glasses containing a minimum concentration ofhigh-Z materials like lead could also be employed if visual observationinto the chamber were desired. The two arrangements in FIG. 6 areconsidered typical examples. A number of variations in geometry, andselection optics, target/spectrum insertion/removal could be consideredfor specific applications. For example prisms also may be used.

Other applications of the IEC x-ray source 601 involve x-ray imaging.Such techniques for using soft x-rays are well-known, e.g., I. H.Hutchinson, Principles of Plasma Diagnostics, Cambridge UniversityPress, N.Y., (1987). A typical approach for adapting the IEC to this useis illustrated in FIG. 7. In this figure, the x-rays 600 are passedthrough a conventional pinhole camera system 604, the image beingrecorded on a detector 605 as shown or on photographic film. The subject603 being photographed would be placed in the x-ray path in theappropriate position desired to obtain the focal length. The subjectwould be sufficiently thin that x-ray transmission through it would bepossible. An x-ray window is used in the arrangement illustrated inanalogy with FIG. 6b. However, if a vacuum arrangement is desired, ageometry similar to FIG. 6a could be employed.

The foregoing characteristics of the bremsstrahlung effect in a plasmacan be the basis for the proper selection of parameters in an IEC devicesuch that a turnable x-ray source can be achieved. As seen in FIG. 2, inan IEC-SS system 400 electrons from electron emitters 409, which areheated by application of an electric current of 1A to 15A at a drivingvoltage of 5-15V, from a source 410, are accelerated 10's keV up to 100keV by a spherical anode grid 401 that is disposed within a sphericalvacuum confinement vessel 402, which also serves as a cathode. Thespherical wire grid 401 is a self-supporting structure, free frominternal supports, having a plurality of openings through whichelectrons may flow. The grid also may be formed of a plurality of vanes,joined together in a geometric pattern which provides a thin profilewhen viewed in a radial direction in order to achieve a high geometrictransparency. Due to the spherical convergence, the energetic electrons403 collide in the center of sphere 404. The interactions between thehigh energy electrons create intense x-rays. The x-ray spectra aredependent on the electron energy controlled by the grid bias 405. Thex-rays are directed to a window 406 in a wall of the vessel andtransmitted via a cylindrical passage 407 to a detector 408. Within thepassage or at other convenient locations in the path of the X-rays, ameans for narrowing the spectrum of the x-rays could be disposed. Suchmeans could be a device using Bragg reflection from a crystal surface,diffraction gratings, prisms, or the like. The IEC-SS makes possible thegeneration of x-rays using relatively low-energy electrons. The IEC-SShas a number of potentially unique and attractive features which mayserve a variety of applications. These features include compactness,relatively low cost, tunability, high photon energy operation. Therelatively narrow natural line-width associated with the IEC-SS canprovide less unusable radiation which could damage optics and targetsamples. In addition, by varying the electron pulse energy in an IEC-SSpulsed mode, chirped x-ray pulses may be generated. The pulse structure,tunability and high photon energy capability of the IEC-SS may providean important tool for studying ultra-fast phenomena. Furthermore, therelatively low cost and compactness of a IEC-SS can make synchrotronlight sources more readily available to users.

Extended x-ray absorption fine structure (EXAFS), which is a powerfultool for structural determination in the materials, biomedical, and manyother scientific fields, has been studied usually at synchrotronradiation (SR) facilities, so far. The development of instruments forEXAFS measurements in a laboratory is important because of theircomplementary usefulness for experiments with SR, especially whenspecial sample preparation and/or quick feedback of the analysis arerequired. The problem with EXAFS measurements performed in a laboratoryis mainly the degradation of spectrum caused by strong characteristicx-ray lines from the source. It is important to develop an x-ray sourcefor dedicated use in EXAFS experiments. So far, the x-ray sources havebeen mostly used for x-ray diffractometry. Therefore, the electron gunis usually designed to operate at high tube voltage to provide strongcharacteristic x-rays. On the contrary, an EXAFS experiment requiresintense continuum x-rays. The use of laboratory base IEC-SS mayalleviate e this problem.

One practical application of the e IEC-SS x-ray beam is to significantlyenhance the imaging ability of low concentration of trace elements inthe human body. Specifically, it could be used in digital differentialangiography (DDA), a new medical x-ray diagnostic concept. P. R. Moran,et al, Physics Today, July (1983); also in “Optics Today,” edited by J.N. Howard (AIP, New York, 1986), p. 308. This new technique is adifferential x-ray absorption diagnostic procedure for imaging bloodvessels. In conventional angiography, x-ray imaging of blood vessels isachieved by intravenously injecting an x-ray absorbing substance such asiodine. The available x-rays used for imaging are extremely broad bandand large doses of both iodine and x-rays are required. A tunable x-raybeam, using a differential x-ray absorption technique, would be a verysensitive diagnostics tool for measuring low concentrations of iodine ata reduced radiation dose. Iodine has a K-edge absorption at a photonenergy of ˜33 kev. In DDA, two x-ray beams are used: on at 33 kev(energy for peak absorption in iodine) and the other at ˜30 kev. Themass attenuation coefficients for these two photon energies differ by afactor of ˜8. The photon flush through the tissue is proportional to theexponent of the mass attenuation coefficient times the mass thickness ofthe tissue. Therefore, the difference between the 33 kev photon imageand the 30 kev photon image is a direct and sensitive measure of theconcentration of iodine, while the images of the bones and other tissuesnot containing the iodine is suppressed. This differential x-rayabsorbing technique would use much lower concentrations of iodineinjected “noninvasively” into the heart via the bloodstream. The imagingand subtraction of the two x-ray beams would be performed at the sametime and, therefore, patient movement during the imaging process wouldnot be a factor.

While the present invention has been described in connection withseveral preferred embodiments, the invention is not limited thereto, andits scope is to be defined by the following claims.

What is claimed is:
 1. A device for generating wave-length tunablex-rays comprising: cathode means comprising a spherical vacuum vessel;anode means comprising a highly transparent spherical grid defining acentral spherical volume, said anode means being centrally disposedwithin said cathode means; electron means comprising a plurality ofelectron emitter/extractor assemblies placed symmetrically near aninternal surface of said cathode means for emitting electrons into saidvessel; means for creating a negative atmospheric pressure in saidspherical vacuum vessel; means for circulating electrons emitted by saidelectron emitter assemblies a plurality of times through a centralregion formed at the center of said spherical vacuum vessel; means forfocusing and converging high energy electrons in said central region;means for applying an electric potential to said anode means whereby thepotential between the anode and cathode means causes the electrons toaccelerate towards the anode means, the increased energy electronsinteracting with idle electrons in said central region thereby producingx-rays, whereby electrons at different energy levels produce x-rays atcorrespondingly different wave-lengths.
 2. The device as set forth inclaim 1 wherein said electron emitter/extractor assembly furthercomprises means for applying electric current to heat said electronemitter/extractor assemblies.
 3. The device as set forth in claim 1wherein said electron emitter/extractor assembly further comprises meansfor applying electric potential to said electron emitter/extractorassemblies to extract electrons.
 4. The device as set forth in claim 1wherein the spherical grid is a self-supporting structure, free frominternal supports, having plurality of opening through which electronsmay flow.
 5. The device as set forth in claim 4 wherein said gridcomprises at least one of a wire and a vane structure.
 6. The device asset forth in claim 1 comprising at least 2 electron emitter assemblies.7. The device as set forth in claim 6 comprising two to eight electronemitter assemblies.
 8. The device as set forth in claim 1 wherein theelectron emitter/extractor assembly comprises an electron emitter and anelectron extractor.
 9. The device as set forth in claim 2 wherein saidcathode means is held at ground potential and the anode means is aconductive electrode having power leads connected, passing through thespherical vacuum vessel and insulated therefrom and being connected tothe means for applying electric potential.
 10. The device as set forthin claim 1 wherein said spherical vacuum vessel is hermetically sealedmetallic shell.
 11. The device as set forth in claim 1 wherein the meansfor applying electric potential to said anode grid provides a positivepotential within a range between 1 Kv to 150 Kv.
 12. The device as setforth in claim 1 wherein the means for applying electric potential toanode grid employs driving currents within a range of 1 Ma to 100 Ma.13. The device as set forth in claim 1 wherein said grid has a geometrictransparency of greater than or equal to 95%.
 14. The device as setforth in claim 1 wherein the pressure in said spherical vacuum vessel isless than 10⁻⁶ Torr.
 15. The device as set forth in claim 1 wherein saidelectric potential to the anode means is within a range of positivepotential between 50 V to 300 V.
 16. The device of claim 3 wherein saidmeans for applying an electric potential to said electron extractoremploys driving currents varying in a range between 10 Ma to 5 A. 17.The device in claim 2 wherein the means for applying electric current toheat said electron emitter provides a current that varies within a rangebetween 1 A to 15 A inclusive.
 18. The device in claim 3 wherein themeans for applying electric current to heat said electron emitteremploys a driving voltage that varies within a range varying from 5 V to15 V inclusive.
 19. The device in claim 1 wherein said grid comprises atleast one of a wire and a vane structure, whereby the profile whenviewed in any radial direction will be thin and thus achieve highgeometric transparency defining a central spherical volume.
 20. Thedevice in claim 1 further comprising means for narrowing the spectrum ofsaid produced x-rays.
 21. The device of claim 20 wherein said meansutilizes at least one of defraction gratings or prisms.
 22. The deviceof claim 1 further comprising means for having a target for said x-raysdisposed within said vacuum vessel.
 23. The device of claim 1 furthercomprising a window for permitting x-rays to escape from said vacuumvessel.
 24. The device of claim 23 further comprising means for having atarget for said x-rays disposed outside of said vacuum vessel.
 25. Thedevice of claim 24 further comprising a camera system for receivingx-rays passing through said target.
 26. The device of claim 22 furthercomprising a camera system for receiving x-rays passing through saidtarget.